Remember after that last nuclear test in September, the one which measured 6.3 on the Richter scale, there was a secondary earthquake about 8 minutes later. Experts suggested that was the result of an underground collapse following the blast. The mountain under which North Korea had set off the 100 kiloton nuke was literally falling apart.

Today, Japanese media is reporting that there was another big collapse earlier this month, with a tunnel under the mountain caving in and killing 100 workers. Another 100 workers were sent in to get them and they were killed by another collapse. From Fox News:

The collapse at the Punggye-ri test site on Oct. 10 occurred while people were doing construction on the underground tunnel, Japan’s Asahi TV reported, citing a source in North Korea. The television station also said North Korea’s sixth nuclear test on Sept. 3 most likely caused the tunnel to crumble and created serious damage in the region.

No officials have confirmed the Japanese TV station’s claims, but experts have feared for more than a month that the test site was on the verge of crumbling since the nuclear blast.

Note that Fox News is pegging this collapse to October 10th while other stories about the same incident suggest the date is unknown. In any case, about 10 days ago the Washington Post published a story about the North Korean test site which suggested it may be suffering from “tired mountain syndrome.”

Images captured by Airbus, a space technology company that makes Earth-observation satellites, showed the mountain literally moving during the test. An 85-acre area on the peak of Mount Mantap visibly subsided during the explosion, an indication of both the size of the blast and the weakness of the mountain.

Since that day, there have been three much smaller quakes at the site, in the 2- to 3-magnitude range, each of them prompting fears that North Korea had conducted another nuclear test that perhaps had gone wrong. But they all turned out to be natural.

That has analysts Frank V. Pabian and Jack Liu wondering if Mount Mantap is suffering from “tired mountain syndrome,” a diagnosis previously applied to the Soviet Union’s atomic test sites.

The danger isn’t just to North Korean construction workers who are probably trying to prepare the mountain for another, future test. The real worst-case scenario is that the mountain collapses in on itself releasing some radioactivity from previous tests into the atmosphere. If that happens, the radioactivity could drift over a significant part of the hemisphere, possibly over Russia and China. Wang Naiyan, former chairman of the China Nuclear Society, told the South China Post, “If the mountain collapses and the hole is exposed, it will let out many bad things.”